How powering PIC in auto ?

[fratello]

In previous post VR1 is not correct...

[Gusse]

Hi Fratello,

Can you measure broken PIC's if the IO's have been damaged?
- are input resistances (IO to GND) at the same level as with new PICs?

Is there any possibility that ESD would enter the system when swithes are pressed?

In car environment, you can easily charge up to very high potentials and if you discharge it to switches, PIC it can be damaged.

1) You could add serial resistors to PIC inputs 4 & 5. Already small resistors (~100 ohm) will damp ESD currents.
2) 5.1V zeners could be added next to C1 and C2.
3) Or do both

BR,
-Gusse-

[fratello]

Thank You for reply !
I have just one damaged 12F675; the resistance from pin 4 to GND are about 9k7, from pin 5 to GND are 9k1, with PIC powering with 5V ; PIC outside from module : pin 4 to GND = 16k3 and pin 5 to GND 13k3. What reveals that ?
Unfortunate I don't have now one good 12F675 for measuring the IO resistance; I wait for postal messenger from e-commerce.

[Gusse]

Sounds pretty much broken to me. You could expect ~100kOhm input resistance or in that range (or even higher) for good parts.

Try to add small resistors and 5.1V zener for switch inputs.

BR,
-Gusse-

[Normnet]

I use a Digi-key 497-6469-1-ND Transient Voltage Suppressor
on every outside lead connected to the circuit.
I believe they are much faster than old zener clamps.
Also try to add a 100 ohm resistor or better ahead of each TVS so it doesn't
have to pull the spike down in the entire system.
For the power supply their could also be a diode and a 100 ohm resistor in
series followed by a large capacitor and TVS in parallel to ground ahead of
the volt regulator.
The 100 ohm resistor can get hot depending on the current draw.
You may have to adjust to less than 100 ohm to maintain voltage at peak
current draw.

Norm

[fratello]

@ Normnet : Can You figure that on my schematic ? Thank You !

[Gusse]

If you are using surface mounted components (0603 or smaller), then it would be more wise to put ESD protection before any serial resistor. Resistor will burn or change value if you force ESD current to go thru it.
Old thru mounted comps can tolarate more ESD, but even with those I would recommend to do the same.

For ESD protection you can use: TVS, varistor or zeners. All those will give appropriate protection, if you have done your PWB layout correct way. Try to minimize all paracitic impedances (inductances & resistance) from ESD current paths.

BR,
-Gusse-